The present invention relates to servo loops in information storage devices. In particular, the present invention relates to the frequency response and linearity table of a servo loop used to position a head over a recording medium.
In an information storage device, such as a disc drive, a servo loop is often used to position a recording or reading head over a medium. The servo loop typically includes an actuator that moves the head in response to a command signal. The command signal is generated by servo logic within the servo loop and is based on a control signal from a host and a position signal from the head. The control signal represents the host's desired position for the head and the position signal represents the current location of the head over the medium. Ideally, the command signal should move the head from its current position towards the desired position.
Before the servo loop can function in this manner, the relationship between the position signal and the actual position of the head over the medium must be recorded within the servo logic. This allows the servo logic to determine the actual position of the head based upon position signals later produced by the head. Ideally, the relationship between the head's actual position and the position signal is linear across a track such that a change in position results in a proportional change in the position signal and the proportionality between the actual position and the position signal remains constant across the track. In practice, the relationship between the actual position and the position signal is not constant; it varies depending on the position of the head. These variations not only make it difficult to determine the actual position of the head from the position signal but also cause the frequency response of the servo loop to vary depending on the position of the head.
A disc drive's frequency response represents its ability to respond to changing input signals. The frequency response of the disc drive is generally controlled by the servo loop and can be defined in terms of a servo loop gain, which represents the servo loop's response to an input signal, and a crossover frequency, which is the frequency at which the gain drops to one. The servo loop gain can be measured by taking the ratio of a first servo loop signal to a second servo loop signal, where the two signals are taken from opposite sides of a point where an external input signal is introduced into the loop.
The frequency response of the servo loop is affected by non-linearities in the relationship between actual head position and the position signal because one factor that affects the overall gain of the servo loop is the amount of change produced in the position signal when the head's actual position changes. Thus, because of variations in the relationship between actual head position and the position signal the overall servo loop gain varies across a track. Since the servo loop gain determines the frequency response of the servo loop, variations in the gain cause position dependent variations in the servo loop's frequency response.
Prior art systems have attempted to minimize the problems associated with these variations by using a laser to precisely measure the position of the head on the track while recording the value of the position signal. The actual position measured by the laser and the recorded value of the position signal are stored in a table that contains entries for a number of positions across each track. The table, known as a linearity table, is stored in a memory location within the servo logic when the storage device is activated and is accessed by the servo logic while the servo loop attempts to position the head. The linearity table allows the servo logic to compensate for non-linearities in the relationship between actual head position and the position signal. The table thereby allows the servo logic to more accurately position the head.
Although the process of taking such laser measurements is fairly accurate, it is also time consuming. This makes it undesirable because it increases the time and cost of producing a disc drive. In addition, it does not fully address variations in the frequency response of the servo loop because it does not measure the frequency response of the servo loop. The present invention provides solutions to these and other problems, and offers other advantages over the prior art.